The present invention relates to a foamed thermoplastic resin sheet formed under a reduced pressure and a method for manufacturing the same.
A foamed thermoplastic resin sheet (hereinafter sometimes referred to simply as xe2x80x9csheetxe2x80x9d) has many advantageous properties such as being light in weight, being superior in shock absorption, and having a thermal insulation effect, and has conventionally been used as a thermal insulating material, a packing material, and the like.
Well known as such a foamed thermoplastic resin sheet is a sheet in which cells grow in either an extrusion direction or a width direction of the sheet, that is, in a direction orthogonal to a thickness direction of the sheet, as disclosed by the Japanese Publication for Laid-Open Patent Application 231745/1996 Tokukaihei 8-231745).
On the other hand, well known as a method for manufacturing a foamed thermoplastic resin sheet is a method in that a sheet is obtained by melting and kneading a thermoplastic resin and a foaming agent in an extruder, and extruding the obtained mixture to the air through a die, as disclosed by the foregoing Tokukaihei 8-231745.
Incidentally, in the present specification, a thermoplastic resin expanded under a reduced pressure at a first expansion stage is referred to as xe2x80x9cfoamy thermoplastic resin,xe2x80x9d and the foamed thermoplastic resin further expanded under a further reduced pressure to a completely expanded state, obtained after or immediately before solidification, is referred to as xe2x80x9cfoamed thermoplastic resin.xe2x80x9d A thermoplastic resin in a state of being expanded under a reduced pressure is to be classified as the former xe2x80x9cfoamy thermoplastic resin.xe2x80x9d
Incidentally, in a field where the foamed thermoplastic resin sheet is for example used as a building material, a thick foamed thermoplastic resin sheet is desired. However, the conventional foamed thermoplastic resin sheet in which cells are grown in an orthogonal direction to the thickness direction is difficult to be formed thick, therefore cannot satisfy the aforementioned requirement.
Besides, in the case where the sheet is formed by the conventional manufacturing method in which the sheet is obtained by extruding a kneaded mixture of a thermoplastic resin and a foaming agent to the air under the atmosphere pressure so that it is foamed, cells in the sheet thus formed grow in a direction orthogonal to the thickness direction of the sheet. Therefore, a foamed thermoplastic resin sheet with a great thickness cannot be obtained.
Then, as a method which has such inconveniences solved, a method in which the foamy thermoplastic resin material extruded from an extruder is passed through a vacuum device so that the foamy thermoplastic resin is further expanded has been practiced.
For example, the Japanese Publication for Laid-Open Patent Application No. 54215/1990 (Tokukohei 2-54215) (the Japanese Patent No. 1639854) discloses an arrangement wherein a roll-like haul-off machine is installed in a vacuum chamber so that the sheet-like foamy thermoplastic resin material is extruded through a die to the vacuum chamber so as to expand and the material thus foamed is hauled by the haul-off machine.
The Japanese Examined Patent Publication 29328/1983 (The Japanese Patent No. 1199174) discloses an arrangement in which a sealing member for ensuring reduction of the pressure in the vacuum chamber is provided at an outlet and a roll-like haul-off machine is installed in a vacuum chamber so as to haul a sheet-like foamy thermoplastic resin material which is extruded through a die to the vacuum chamber thereby expanding.
The manufacturing method disclosed by Tokukohei 2-54215, however, has a drawback in that installment of the haul-off machine in the vacuum chamber causes the sealing mechanism in the vacuum chamber to become complicated, and makes the manufacturing device bulkier.
Further, regarding the manufacturing method disclosed by Tokukosho 58-29328, since the foamed sheet, while being hauled by the haul-off machine, pushes the sealing member at the outlet of the manufacturing device, cells in the foamed sheet are crushed and surfaces of the sheet are scarred. Note that the drawback in that cells of the foamed sheet are crushed is more remarkable in the case of Tokukohei 2-54215.
Therefore, though usually cells growing long in the thickness direction of the sheet are obtained in the case of expansion under a reduced pressure, such an effect of growth of cells due to pressure reduction is not sufficiently achieved in the foregoing prior art, and cells emerging in this case are cells growing long in the direction ort hogonal to the sheet thickness direction, that is, in the sheet width direction or in the extrusion direction, like in the aforementioned case. Presence of a number of such cells in a sheet leads to a problem that the sheet cannot be made thicker.
The present invention is made in light of the aforementioned problem s, and the object of the present invention is to provide a foamed thermoplastic resin sheet which has a high foaming ratio and is formed thick, as well as to provide a manufacturing device which is capable of producing this sheet. To achieve the foregoing object, a foamed thermoplastic resin sheet of the present invention is characterized in having a foaming ratio of not less than 2.5, and in that cells, existing in an interior part which extends inward from a depth of 20 percent a whole thickness of the sheet from the front and rear surfaces thereof respectively in a thickness direction thereof and from 15 percent a width of the sheet from both side edges respectively, satisfy the following expressions (1) and (2):
0.5xe2x89xa6D/Cxe2x89xa60.9xe2x80x83xe2x80x83(1) 
0.5xe2x89xa6E/Cxe2x89xa60.9xe2x80x83xe2x80x83(2) 
where C represents a mean cell diameter in the thickness direction of the foamed thermoplastic resin sheet, D represents a mean cell diameter in an extrusion direction of the same, and E represents a mean cell diameter in the width direction of the same.
The inventors of the present invention eagerly studied to achieve the foregoing object, and found that a foamed thermoplastic resin sheet which is thick, has a high foaming ratio, has excellent smoothness of surfaces, and an excellent appearance could be obtained in the case where the foaming ratio was not less than 2.5 and the mean cell diameters in the thickness, extrusion, and width directions of the foamed thermoplastic resin sheet in the interior part of the sheet, other than the peripheral parts as described above, satisfied the foregoing requirements (1) and (2). The present invention was completed based on this result of study.
With the foregoing arrangement, a foamed thermoplastic resin sheet which meets users"" demands, i.e., which is characterized with a great thickness and a high foaming ratio, can be provided.
Furthermore, a manufacturing method of a foamed thermoplastic resin sheet of the present invention is characterized by comprising the steps of (1) melting and kneading a thermoplastic resin and a foaming agent, continuously extruding a foamy thermoplastic resin as a melt/kneaded material through a die into a sheet form, and hauling the sheet-like foamy thermoplastic resin material through a vacuum chamber having a pair of wall surfaces which face each other in a thickness direction of the resin material with a gap therebetween greater than the thickness of the resin material, (2) narrowing the gap between the wall surfaces in pair to the thickness of the resin material, so as to make the vacuum chamber ready for pressure reduction, and (3) reducing a pressure in the vacuum chamber, while increasing the gap between the wall surfaces in pair to a desired thickness of the foamed thermoplastic resin sheet to be formed, so as to continuously produce the foamed thermoplastic resin sheet.
With the foregoing arrangement, the melted/kneaded mixture of the thermoplastic resin and the foaming agent is extruded through the die by an extruding operation by the extruder, thereby becoming a sheet-like foamy thermoplastic resin material. The foamy thermoplastic resin material is further expanded under a reduced pressure in the vacuum chamber, thereby becoming the formed thermoplastic resin sheet.
Here, the thickness of the foamed thermoplastic resin sheet is determined by a gap between a pair of the wall surfaces facing each other in the vacuum chamber. In other words, the foamed thermoplastic resin sheet is expanded to a thickness equivalent to the gap between the wall surfaces in pair. Therefore, by changing the gap between the wall surfaces in pair, foamed thermoplastic resin sheets with various thicknesses can be obtained.
Furthermore, in the step (2) in which the foamy thermoplastic resin material expands due to the reduced pressure in the vacuum chamber, the gap between the wall surfaces in pair in the vacuum chamber is widened in accordance with the thickness of the foamed thermoplastic resin sheet to be formed. Therefore, in the case where the vacuum chamber is sealed with, for example, sealing members, the position relationship between the foamy thermoplastic resin material and the sealing members (before expansion due to pressure reduction) and the position relationship between the foamed thermoplastic resin sheet and the sealing members (after expansion due to pressure reduction) can be made to coincide. Therefore, soft and flexible materials can be used to form the sealing members, and a problem that sealing members crush cells in the foamed thermoplastic resin sheet can be avoided. This ensures that the vacuum chamber keeps a set desired pressure, without applying a great force from the sealing members to the foamy thermoplastic resin material and the foamed thermoplastic resin sheet at all times since the start of pressure reduction. As a result, it is possible to easily obtain a foamed thermoplastic resin sheet which maintains a good expanded state under the reduced pressure thereby having a high foaming ratio and being formed thick.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.